In the 1800's when "risibility" was a relatively popular word, "risible" was understood to mean "having the power to create laughter", and, by extension, "creating laughter". "Risibility", however, was understood to mean not "creating laughter" but "the quality of laughing". A critical pronouncing dictionary
–
MετάEdFeb 6 '12 at 15:17

I think these (risibility, humorousness) words are not so usable because the humor is always two-sided thing. You can't make a joke that would be funny for everybody. Even more, jokes are very rarely funny for really broad public. Children jokes are a bit more universal, but they are for children only. The jokes for elder public highly depend on the culture and the place of the listener in it.

Because of that the word humor is used instead. It supposes the sense of humor on both sides.

So, why not Philosophical Theories of Humor instead? You won't be risible with such title :-)